In recent years, a segment friction material is under development as a wet friction material in order to improve material yield with resultant cost reduction and to lessen drag torque with resultant high fuel efficiency in a vehicle. The segment friction material has a flat ring shaped metal core (core plate) and segment pieces made of friction material substrates cut into a segment piece shape along a circumference of the flat ring shape of the metal core. A set of the segment pieces is arranged and joined with an adhesive on one surface of the metal core side by side along an entire circumference of the metal core with an interval to be an oil passage. Another set of the segment pieces made of friction material substrates cut into a segment piece shape is joined on a rear surface of the metal core with an adhesive, too. Such segment friction material is applicable to a friction material clutch device that has a single or plural friction plates for use in an automatic transmission, which may be hereafter referred to as “AT”, of automobiles or motorcycles or the like.
As an example, a wet hydraulic clutch is used for an automatic transmission of an automobile or the like. The wet hydraulic clutch has a plurality of segment friction materials and a plurality of separator plates laid alternately on one another. Both the plates are contacted with a hydraulic pressure to transmit a torque. Lubricating oil or automatic transmission fluid, which may be hereafter referred to as “ATF”, is supplied to between both the plates for the purpose of absorbing frictional heat generated when they are shifted from a disengaged state to an engaged state or preventing abrasion of the friction material and the like. “ATF” is a registered trademark of Idemitsu Kosan Co., Ltd.
However, a distance between the segment friction materials and the separator plates associated therewith is set small in order to increase response of the hydraulic clutch. In addition, a total area of an oil passage provided on the segment friction material is restricted in order to assure a sufficient torque transmission capacity when the hydraulic clutch is engaged. As a result, there were problems that ATF remaining between the segment friction material and the separator plate was hardly discharged when the hydraulic clutch was disengaged and drag torque by ATF was generated depending on relative rotation of both the plates.
In order to solve such problems, a patent document 1 discloses a wet friction plate for the purpose of reducing a drag torque by promoting discharge of ATF while maintaining cooling capacity by ATF. The wet friction plate comprises a core plate made of an iron-based metal plate and a plurality of friction materials (segment pieces) concentrically arranged in two rows on both top and rear surfaces of the core plate to form a first groove extending in a radial direction of a metal core and a second groove extending in a circumferential direction of the metal core. Therefore, generation of a drag torque when torque transmission is not required can be prevented while maintaining cooling capacity by lubricating oil.
In addition, a patent document 2 discloses an inner clutch plate and a driving force transmission using the same. The inner clutch plate comprises a flat ring shaped metal core and two ring shaped friction materials concentrically arranged on a top surface of the metal core at an interval. Each of the ring shaped friction materials has a plurality of circumferential grooves and a plurality of radial grooves for communicating from an inner edge or an outer edge of the flat ring shaped metal core to the circumferential grooves provided. Thereby, more lubricating oil can be introduced between the inner clutch plate and an outer clutch plate that is in frictional engagement therewith. A dynamic pressure thereof can improve an effect to separate the clutch plates from each other. As a result, a drag torque can be more effectively reduced.
PATENT DOCUMENT 1: Japanese Patent Laid-Open Publication No. 2003-090369
PATENT DOCUMENT 2: Japanese Patent Laid-Open Publication No. 2008-106929